In a gravity thickener, the depth of the sludge blanket varies with temperature, and higher temperatures require more-shallow blankets to promote compaction thickening.

As a rule, positive-displacement pumps create more shear on solids. This shear is why positive-displacement pumps are not preferred for thickening and dewatering.

Multiple Choice Questions:

What is a possible reason for a gravity thickener not performing well?

A. Overflow rate is too high
B. Thickened sludge pumping rate is too high
C. Short-circuiting of flow through tank
D. All of the above

What is the normal retention time range for primary sludge in a gravity thickener, depending on the temperature?

A. 0.5 to 1 days
B. 1 to 2 days
C. 2 to 3 days
D. 3 to 5 days

If septicity (floating solids, foul odors, and reduced underflow currents) occurs after solids have been stored too long in a gravity thickener, which of the following can be done to help address the problem?

A. Elutriation of the thickener feed
B. Aeration of the thickener feed
C. Addition of chlorine or hydrogen peroxide
D. All of the above

What should be the solids feed temperature range in a gravity thickener operation if the secondary-to-primary sludge ratio ranges between 4:1 and 6:1?

Questions developed by LaShell Stratton-Childers and reviewed by the Operations forum editorial advisory board and the Association of Boards of Certification (Ankeny, Iowa) Wastewater Treatment Validation and Examination Committee.

3. True. Because thickening is dependent on the weight of the solids, one on top of the other, a blanket of solids in the thickener is necessary to promote compaction thickening. The depth of the blanket varies with temperature. Higher temperatures require more-shallow blankets.

4. False. Positive-displacement pumps create less shear on solids. Because shear is presumed to be detrimental to thickening and dewatering, positive-displacement pumps are preferred.

5. d.

6. b. Gravity thickener retention time ranges between 1 and 2 days for primary sludge. The exact time depends on the temperature.

7. d.

8. c. A temperature range between 59°F and 69°F (15°C and 20°C) is acceptable if the secondary-to-primary sludge ratio ranges from 4:1 to 6:1.